In recent years, light sources which illuminate a display surface from the rear side (backlights) have been commonly used in liquid crystal display devices for personal computers, OA devices, liquid crystal televisions, or the like, and illumination devices such as signboard lamps. Of these, a direct backlight is known which comprises a plurality of discharge lamps arranged on a reflector plate and a diffuser plate which is disposed over the discharge lamps.
In the field of liquid crystal display devices, there are demands for large screen size, high luminosity and uniformity. Consequently, there is a tendency for the number of discharge lamps used per device to increase, and for the tube voltage of the discharge lamps used to operate at higher voltage. For example, in a 32-inch backlight using CCFL (Cold Cathode Fluorescent Lamps), the tube voltage is around 1 kV rms. Therefore, it is not possible to ignore the effects of the parasitic capacitance between the high impedance load and the housing, a bias occurs in the luminosity distribution of the discharge lamps due to the effects of current leaking to the housing, and hence there is a problem in that the luminosity becomes non-uniform.
Therefore, a possibility might be to use hot cathode fluorescent lamps (HCFL) which have a higher output and lower tube voltage than CCFL. If HCFL are used, then it is possible to reduce the number of discharge lamps and to reduce the operating circuits, compared to CCFL. Furthermore, since the tube voltage is low and the leakage current flowing in the parasitic capacitance between the discharge lamps and the housing is low, then there is little bias in the luminosity of the discharge lamps. Moreover, since noise is low, then there is also little effect on peripheral circuits.
However, in a discharge lamp operating device which operates HCFL at high frequency by means of an inverter circuit, if the filament of a discharge lamp becomes disconnected, for instance, when in a loadless state (a state where the discharge lamp is removed) or at the end of the lifespan of the lamp, or the like, then if the oscillation of the inverter circuit is continued, a high voltage will occur between the output section and the socket section, and there is a risk of danger, such as electric shock. Therefore, in the event of an abnormality such as that described above, it is normal to halt the oscillation of the inverter circuit compulsorily.
Patent Document 1 discloses a composition in which, in order to preheat a pair of filaments of a discharge lamp, a resonance capacitor (capacitor C1) is connected between the non-power source side terminal of one filament and the non-power source side terminal of the other filament, the voltage across the stem of the discharge lamp is detected, and if this voltage has exceeded a prescribed value which indicates disconnection of the filament, then the inverter circuit is halted or the output thereof is reduced.
In a discharge operating device in which a resonant capacitor also serves as a preheating capacitor, as in Patent Document 1, the current in the filaments is determined by the resonance characteristics. However, a filament has characteristics whereby the resistance value becomes greater when the filament heats up. Consequently, if a fixed current is passed through a filament, the voltage becomes greater in accordance with the resistance value, and even if a filament is in a normal condition, the voltage across the stem varies greatly. Consequently, the detection threshold value of a comparator for detecting the voltage across the stem must be set higher than the range of variation of the voltage across the stem in a normal state. Accordingly, in the discharge lamp operating device in Patent Document 1, there is a problem in that the accuracy of detecting disconnection of the filaments is low.
If discharge lamps are used as a backlight for a liquid crystal television, then reduction in the thickness of the backlight is desired, and there is a tendency for the discharge lamp tubes to be become finer. Moreover, in a liquid crystal television, long lifespan of the backlight is desirable, and the preheating conditions of the filaments are subjected to strict restrictions. Therefore, it is desirable to use a discharge lamp operating device which is able to designate the preheating current of the filaments, independently of the resonance characteristics of the discharge lamp operating device.
Patent Document 2 discloses a discharge lamp operating device based on a winding preheating system which passes a preheating current through a filament by using a preheating transformer, in which a direct current is passed through the filaments of the discharge lamps, and disconnection of a filament is detected by the presence or absence of this direct current. In this discharge lamp operating device, it is possible to designate the preheating current independently of the resonance characteristics, but since the preheating current of the filaments varies with the resistance value of the filaments, then there is little voltage change across the stem.
However, in the discharge lamp operating device shown in Patent Document 2, since a direct current is passed through the filament, then a resistance is connected between the DC power source unit and the filament. In this case, if stresses, such as the starting voltage of the discharge lamps, and the like, are taken into account, then it is necessary to arrange a plurality of resistances in series, and there is a problem in that the number of components becomes greater.
Furthermore, if the direct current power source section and the discharge lamp load need to be isolated, then a separate power source for detecting disconnection of the filaments becomes necessary and there is a problem in that the number of components becomes greater.